Conventional horizontal skylight suffers from poor sunlight collection when the sun is low in the sky, i.e., when the sun's elevation angle is small. This poor low-sun-angle performance leads to poor lighting in the wintertime in most moderate latitudes, and to poor lighting early and late in the day in all locations. Previous attempts to solve this problem have sometimes used fixed reflectors or prismatic lenses above the skylight penetration with less than adequate performance. None of these previous attempts to solve the low sun angle inadequate sunlight collection problem have proven successful commercially.
Conventional horizontal skylights also suffer from excess sunlight collection near solar noon in the summer months, i.e., when the sun's elevation is high. This excess high sun angle performance leads to large solar heat gain in to the building below the skylight during peak air conditioning season, leading to very high peak cooling loads, which require greater air conditioning system capacity and investment, as well as higher electricity consumption and cost. Previous attempts to solve this problem have sometimes used expensive and complicated skylight blinds beneath the skylight, or an expensive electrochromatic or thermochromic glass layer in the skylight which changes transmission with the application of electrical current or with high ambient temperatures. None of these previous attempts to solve the high sun angle excess sunlight collection problem have proven successful commercially.